1. Field of the Invention
The present invention relates to an electrophotographic organic photoconductor used in a copying machine, a laser printer and the like. More particularly, the present invention relates to a monolayer type electrophotographic organic photoconductor which can be charged with a desired polarity, i.e., either positively or negatively, and has a small residual potential and an improved sensitivity.
2. Description of the Related Art
In an electrophotographic copying machine using a digital optical system, the light source generally has a wavelength of 700 nm or more. As a photoconductor having a sensitivity in this wavelength region, an organic photoconductor (OPC), an amorphous silicon (.alpha.-Si) photoconductor, a selenium photoconductor and the like are known. Among them, the organic photoconductor is mostly used because of its high sensitivity and low cost.
The following two types of organic photoconductors are known: A so-called function separate type organic photoconductor in which a charge generation layer (hereinafter referred to as the "CGL") and a charge transport layer (hereinafter referred to as the "CTL") are laminated onto each other, that is, a multi-layer type photoconductor; and a monolayer type photoconductor having a photosensitive layer including a charge transport material and a charge generation material. The function separate type organic photoconductor mainly used these days comprises the CGL and the CTL successively laminated on a conductive substrate and has a large sensitivity and a high mechanical strength.
The charge transport material used in the photoconductor is required to have high carrier mobility. Since most of the charge transport materials with high carrier mobility are hole transport materials, all the practical organic photoconductors including such a charge transport material are always negatively charged. However, when a negative charge is produced on the surface of such an organic photoconductor, a large amount of ozone is produced due to a reaction with oxygen in air, since the charge is caused by a negative corona discharge, resulting in problems of environmental contamination and degradation of the photoconductor to be obtained. Moreover, a special charging system for preventing the generation of ozone, a decomposition system for the generated ozone, a system for exhausting the ozone from the apparatus and the like are required in order to solve the above-mentioned problems, resulting in a complicated process and system.
The multilayer type photoconductor requires two coating processes for forming the photosensitive layer. Moreover, an interface is present between the CGL and the CTL. Such an interface tends to cause an interference fringe.
As a charge transport material for overcoming the above-mentioned disadvantages, use of an electron transport material is proposed. For example, Japanese Laid-Open Patent Publication No. 1-206349 discloses the use of a compound having a diphenoquinone structure as the charge transport material.
However, the electron transport materials including diphenoquinones generally have a poor compatibility with a binding resin, resulting in lengthening the hopping distance of an electron. Therefore, an electron has difficulty in moving within an electric field with a low voltage, and the residual potential is fairly high. Thus, a photoconductor utilizing a practical electron transport material is desired to be developed.
On the other hand, a photoconductor which can be charged with a desired polarity, i.e., either positively or negatively, has a wider range of application. As a result, the foregoing various disadvantages can be overcome.
Moreover, the monolayer organic photoconductor including the electron transport material and the like dispersed therein can be easily produced and has a number of advantages in the prevention of coating defects and improvement of optical characteristics of the photoconductor.